The technologies in such fields as electric, electronic, office automation (OA), audio-visual (AV), and automotive industries have been making remarkable progress in recent years. Polymeric materials used in such new fields are required to have meritorious properties such as high strength and high heat resistance. They have come to be required to have high level properties such as high dimensional accuracy, strength, rigidity, solder heat resistance and thin-wall processability by the advancement of reduction of the size and wall thickness particularly of electric parts such as relay parts, coil bobbins and connectors. As one of the polymeric materials which can satisfy the requirements, aromatic polyesters are suitably used. Among the aromatic polyesters, particularly liquid crystal polyesters showing anisotropy in molten state have desirable thin-wall processability and are rapidly coming into wide use as a material for electric parts.
Although acetylation method, phenyl esterification method and acid chloride method are known as the methods for producing aromatic polyesters, the liquid crystal polyesters showing anisotropy in molten state are mostly produced by acetylation method, in which polymerization is carried out by solution polymerization in a solvent having a high boiling point or by melt polymerization using substantially no solvent. In acetylation method, an aromatic hydroxy compound, one of the monomers, is converted into an acetic acid ester by the reaction between an aromatic hydroxy compound and acetic anhydride and the acetic acid ester is then polymerized by the intermolecular elimination of acetic acid. The conversion of an aromatic hydroxy compound into an acetic acid ester is generally conducted by adding acetic anhydride in an excess amount of about 1.1 moles per mole of the hydroxyl group and allowing the resulting mixture to react under reflux of acetic anhydride.
However, the preparation of acetic acid phenyl esters by the reaction between an aromatic hydroxy compound having the following formula (I) and acetic anhydride problematically accompanies the occurrence of side reactions such as the replacement of the hydrogen atom attached to the benzene nucleus by an acetyl group or the coloring of the reaction product at the later stage of reaction. ##STR1## wherein R is halogen, alkyl having 1-5 carbon atoms or phenyl; and m and n are each an integer of 0-2, provided that when m is 2, two substituents represented by R may be different from each other.
Therefore, acetic acid phenyl esters used as the monomer for aromatic polyesters having a satisfactorily high purity cannot be obtained by the above-mentioned method. Acetylation method, when used for polymerizing an aromatic polyester having the corresponding repeating unit, cannot sufficiently raise the molecular weight of the resulting polymer nor prevent the coloring of the resulting polymer. Thus, by acetylation method, practically usable polymers are hardly obtained.
The same applies to the case where acetic anhydride is replaced by a carboxylic acid anhydride other than acetic anhydride.